Methacrolein (or methacrylaldehyde) is an industrially useful chemical for the synthesis of a multitude of acrylate based polymers and resins. Among these applications, it is an intermediate for the production of methacrylonitrile and methacrylic acid. The latter is produced in an oxidation process and is a precursor for large scale applications such as methyl methacrylate and other esters.
The known process for methacrolein production involves the formation of propionaldehyde from ethylene hydroformylation (ethylene oxo) followed by the coupling of propionaldehyde with formaldehyde to form methacrolein. The yields for this synthetic approach are often greater than 95%. The major drawback of this process is that it requires a homogeneous Mannich base (typically a primary or a secondary amine) and an organic acid. While earlier patents detailed batch processes for the aldol reaction limiting their industrial applicability, latter patents revealed methods for a continuous process although the need for huge quantities of Mannich base and organic acid requires that the continuous process be run diluted at high atmospheric pressures. Although research in this area has developed the use of solid acid catalysts such as Amberlyst for the aldol condensation of formaldehyde and propionaldehyde, the process still requires one equivalent of a secondary amine. In many cases, this amine catalyst is consumed by the reaction which adds to the separation and feed costs.
The art additionally details various vapor-phase condensation reactions involving formaldehyde and other aldehydes or ketones over an unmodified silica gel catalyst. Although methacrolein was produced in some of this work, there are at least three technical requirements that limit the commercial ability of this process: 1) The ratios of the aldehyde to formaldehyde were not 1:1 and in many examples were as high as 20:1, thus increasing the feedstock costs, separation costs, or both; 2) The need for high temperatures for the condensation reaction, for example, the condensation of propionaldehyde and formaldehyde over the silica gel catalyst required temperatures in excess of 460° C.; and 3) Despite the high temperatures required to give appreciable amounts of methacrolein, the highest recorded conversions were only about 45%.
In an ideal methacrolein process, a continuous 1:1 reactant stream of only formaldehyde and propionaldehyde would be condensed together over a simple regenerable heterogeneous catalyst. This idealized process would aid downstream separations since the catalyst would not be part of the reactor effluent. Noting these advantages, research in developing this type of ideal process has progressed in the direction of designing an efficient heterogeneous catalyst. For example, the prior art teaches the synthesis of methacrolein from acetaldehyde or propionaldehyde and methanol over a BiaMobXcOd catalyst (where, X=Al, Si, Ti, Fe, Co, Ag, W, V and/or P and a, b, c, d are the atomic ratios). Although these 3-metal catalysts had limited examples of acceptable runs with 86.6% selectivity at 67.5% conversion, the drawbacks for successful industrial implementation of this process include the synthesis of the complicated 3-metal catalyst along with the low yields obtained in all cases. Related catalyst work with borosilicate catalysts provided only low single pass yields of 56.6% with 57.5% conversion and 98% selectivity. Nearly 40% of the propionaldehyde used had to be recycled after each pass increasing separation volumes and costs. Similarly in other research, the aldol condensation of propionaldehyde and formaldehyde over a sodium oxide and silicic acid catalyst systems also showed poor yields of only 46%.
In addition to developing an effective regenerable heterogeneous catalyst, being able to supply a clean and reliable formaldehyde source can be problematic. Besides safety concerns, the main complicating issue with formaldehyde as a starting material is that it cannot be isolated in pure form since formaldehyde is notoriously unstable and frequently reacts with itself to form oligomers and polymers of paraformaldehyde. Paraformaldehyde can result from the evaporation of aqueous solutions of formaldehyde and cannot be used as a feed in vapor-phase reactions. The formation of paraformaldehyde can additionally contribute to yield losses and increased maintenance costs as the paraformaldehyde deposits on equipment and piping.
The most common commercial forms of formaldehyde are formalin or the 52% to 55% by weight aqueous solution. It would be preferred to avoid these forms of formaldehyde in the synthesis of methacrolein and/or related α,β-unsaturated aldehydes because of potential issues involving safety, expense, and handling difficulties. Formalin and the 52% to 55% by weight aqueous solutions are toxic and suspected carcinogens. Although trioxane is a convenient form of formaldehyde, it is too expensive for use in a commercial process. The presence of an excessive amount of water or other hydroxylic compounds is undesirable since it would be expensive to separate these components from the desired aldol reaction products.
The present invention teaches that the oxidative dehydration of methanol to make formaldehyde can be used with propionaldehyde in the vapor-phase over an unmodified anatase titania catalyst for the facile synthesis of methacrolein. Moreover, it was surprisingly discovered that an α,β-unsaturated aldehyde such as methacrolein is stable over a titania catalyst given the potential for this molecule to undergo further aldol condensation and oligomerization. The present invention's use of a silver catalyst to convert methanol to formaldehyde and a titania catalyst to synthesize methacrolein and other α,β-unsaturated aldehyde compounds offers multiple advantages not previously addressed by the prior art. These advantages include: 1) a reliable and high quality formaldehyde stream; 2) improved aldol selectivity and conversion; 3) high single pass yields; 4) regenerable catalyst activity; 5) cleaner downstream separations due to the heterogeneous titania catalyst; and 6) efficient reactivity at atmospheric pressures.